Sulfur which was in the form of sulfide by combining with the objective metal remains in the post-leaching residue, which was obtained after leaching of objective metal in the hydrometallurgical process for sulfide ores.
The flotation method is a method for separating and recovering the objective substance by adhering it onto the surface of air bubbles by utilizing the differences of the hydrophilicity and hydrophobicity of the surface of the substances. Since sulfur has hydrophobicity, it can be easily recovered by the flotation method.
For example, Japanese Patent Application Publication No. 2002-53310 (Patent document 1) shows the process for recovering sulfur from the floats in which sulfur has been concentrated by applying the flotation method to the post-leaching residue of zinc concentrate containing sulfur produced in the process of leaching the zinc concentrate.
In case where sulfide ore of the objective metal contains gold, it is difficult to leach gold during the process for leaching the objective metal in an ordinary hydrometallurgical method, and therefore, gold remains in the post-leaching residue together with sulfur which had constituted the sulfide with the objective metal. When the sulfur is recovered by the flotation method, gold also can be recovered together.
For example, Japanese Patent Application Publication No. 2006-57133 (Patent document 2) shows a process wherein the mixture of gold and sulfur recovered by the flotation method is heated to the temperature sufficient for melting of sulfur, and then gold and sulfur are separated by centrifugation.
With regard to the process for treating the gold recovered together with sulfur, a process in which sodium cyanide or sodium thiosulfate is used is generally known as a process for leaching gold. However, cyan has toxicity, and therefore, the safety of waste liquid treatment and environmental burden are problematic. When sodium thiosulfate is used, leach rate of gold is lower compared to the case where cyan is used, resulting in a higher cost.
Therefore, the patent document 2 shows a process wherein a mixture of gold and sulfur is recovered by a flotation, and the sulfur is then separated from the mixture by melting and centrifugation, and is subsequently treated with a pyrometallurgical method. However, a special device is required for performing the centrifugation under heating. Even using the pyrometallurgical method, there is a problem that the environmental burden becomes heavier due to the remaining sulfur.
In Japanese Patent Application Publication No. 2010-180450 (Patent document 3), though no concrete method to remove sulfur is shown, it discloses a process wherein sulfur is removed from the mixture of gold and sulfur recovered by the flotation method and then oxidizing roasting is performed, and further, the product of oxidizing roasting is dissolved in a sulfuric acid solution for separating and recovering contained gold residue. Also in this process, a dry process of oxidizing roasting is necessary.
However, if gold can be leached safely and efficiently from the recovered material from the flotation method, which is an intermediate product containing sulfur and gold obtained by the hydrometallurgical method, it is possible to make the whole process more compact by employing the hydrometallurgical method.